This invention relates to film capacitors made from metallized polymer films and to methods for making film capacitors. The capacitors have increased moisture and breakdown resistance.
Typically, metallized film capacitors include two tightly wound sheets, wrapped around a core. Each sheet includes a dielectric layer and a metallized layer. The metallized layer does not extend to the opposing ends of the sheet leaving a non-metallized margin on opposing sides of each sheet. The ends of the roll formed from the two tightly wound sheets are sprayed with a conductive metal to form a conducting termination for the capacitor. Capacitors made in this way can be used for a variety of purposes depending upon factors such as the type of sheet material as well as the thickness and dielectric constant of the sheet. Typical materials for the sheet are, for example, oriented polypropylene or poly-(ethylene)-terephtalate. The conductive metal termination is typically applied in a vacuum metallizer and is generally comprised of aluminum, zinc or alloys thereof.
A common problem during the manufacturing of metallized film capacitors is the entrapment of air during the winding process. Air gaps inside the capacitors can allow water to penetrate in between the dielectric sheets, corroding the thin metallized layers. This corrosion can reduce the active area of the capacitor and its capacitance. Air entrapment can also cause corona discharges to occur during use. Corona discharges can degrade the dielectric material and the metallized layer, which can result in capacitance loss. Several methods have been devised to improve moisture resistance and reduce the amount of air entrapped in a wound film capacitor.
The most common method of improving moisture resistance is vacuum impregnation. In this process, after the capacitor has been wound and terminated in a spray-coating process, the capacitor is immersed in a bath of dielectric fluid or resin such as epoxy resin, polyurethane resin or proprietary organic resins. Examples of the resins can be found in U.S. Pat. Nos. 4,897,791 and 6,014,308. Exposing the capacitor to a vacuum while in the bath causes the entrapped air to diffuse out of the tightly wound capacitor. The capacitor is then exposed to a higher pressure while still immersed in the bath causing the fluid to penetrate into the minute gaps between the layers. The capacitor is then removed from the bath and the resin is allowed to cure. The resin seals the capacitor, improving its moisture resistance.
However, the penetration of the dielectric fluid into the tightly wound capacitor during vacuum impregnation typically is not complete. Furthermore, the dielectric fluid also penetrates into voids and covers the surface of the spray coated termination, making the welding or soldering of terminals to surfaces of the spray coated terminations difficult or impossible. Impregnation of the capacitor prior to spray termination is difficult, since the dielectric fluids tend to cover the metal exposed on the terminal ends of the metallized layer, making it difficult to achieve sufficient contact during the spray coating process. U.S. Pat. No. 5,043,843 teaches how this problem can be overcome by impregnating the capacitor prior to spray termination and by cleaning the terminal edges through etching with reactive gas plasma.
Wound capacitors that are used in dry applications often face the problem of losing tension on the outer windings. This loss of tension can cause the outer windings to develop gaps which can create the aforementioned problems of air/moisture intrusion and corona discharge. The problem of air/moisture intrusion is not as serious of a problem in oil filled capacitors. The oil fills the gaps in the capacitors effectively sealing the outer windings. Capacitors in dry applications, however, can lose capacitance due to this effect.
Accordingly, a need exists for an improved method of decreasing the entrapment of air and increasing the moisture resistance of a capacitor.
The invention includes capacitor films and capacitors that have increased moisture and breakdown resistance. The capacitor films include a polymer coating that helps prevent air entrapment.
In one embodiment, the capacitor film is a metallized film. The metallized film is a dielectric film with a metal layer deposited on a portion of a surface of the dielectric film. The surface of the dielectric film upon which the metal layer is deposited, has a margin portion, which is free of deposited metal. A layer of heat fusible polymer is coated over the metal layer and margin portion.
Preferably, the dielectric film is made from a heat fusible polymer made from a polymer which has a melting point below or at a curing temperature of the dielectric film. A preferred heat fusible polymer is made from a polyethylene and has a thickness of 0.1 xcexcm to 1.5 xcexcm.
The metallized film can be formed into a capacitor by rolling the film or by stacking the film in layers. Preferably, heat and pressure is applied during the winding or stacking process to exclude air and to fuse the metallized film together. The capacitor can also be exposed to reduced pressure to extract entrapped air and, while under reduced pressure, exposed to heat to cause the heat fusible polymer to fuse the layers of the capacitor.
In another embodiment of the metallized film a layer of heat fusible polymer is placed on a surface of the dielectric film opposite the surface upon which the metal layer was deposited.
In yet another embodiment of the metallized film, the metallized film contains a dielectric film and a first and a second metal layer. The first metal layer is deposited on a portion of the first surface of the dielectric film, and the first surface has a first margin portion which is free of deposited metal. The second metal layer is deposited on a portion of the second surface of the dielectric film, and the second surface has a second margin portion which is free of deposited metal. The second margin portion is located on the opposite edge of the dielectric film from the first margin portion. A layer of heat fusible polymer is deposited on the first surface of the dielectric film, the second surface of the dielectric film or on both the first and second surfaces of the dielectric film.
One embodiment of the method of making a metallized film includes providing a dielectric film, depositing a metal layer onto a surface of the dielectric film, and depositing a heat fusible polymer layer onto a surface of the dielectric film or onto the metal layer.
An another embodiment the capacitor includes a dielectric film layer, a metal layer, a heat fusible polymer layer. A preferred way of making the capacitor is by stacking layers of a capacitor film having a dielectric film layer, a metal layer, and a heat fusible polymer layer or by winding a capacitor film having a dielectric film layer, a metal layer, and a heat fusible polymer layer around a core.